Retirement Benefits - Summary of Components of Retirement Benefit Expense (Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost on benefit obligation	$ 42.5	$ 52.7	$ 49.7
Assumed return on assets	(60.5)	(64.8)	(60.1)
Amortization of prior service costs (credits)	0.1	0.1	0.1
Amortization of net losses (gains)	57.4	40.9	70.7
Retirement benefit expense (income)	39.5	28.9	60.4
Medical and Life Insurance Benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost on benefit obligation	0.8	1.2	1.2
Assumed return on assets	0.0	0.0	0.0
Amortization of prior service costs (credits)	0.0	(0.2)	(0.2)
Amortization of net losses (gains)	(3.7)	(3.8)	(3.8)
Retirement benefit expense (income)	$ (2.9)	$ (2.8)	$ (2.8)